The ultralight aircraft industry has experienced considerable growth in recent years since ultralights are relatively inexpensive compared to single engine general aviation aircraft, easy to fly, usually run on ordinary automotive fuel, and often require only an 80 foot pasture for a runway.
Generally, an ultralight aircraft comprises an open hang cage formed from high grade aluminum tubing in which a pilot sits on a canvas seat. A small engine, such as a two cycle-twin cylinder engine, is supported on engine mountings behind the pilot. A fuel tank is mounted to the hang cage above the engine. The wing, which can be rigid or collapsible (i.e., a Delta or Rogallo type wing such as the type described in U.S. Pat. No. 2,546,078 to G. S. Rogallo et al) is usually king post and cable braced to the hang cage, or strut braced. With this construction, an ultralight is of minimal weight to reduce take off and landing distances while maintaining reasonable power requirements by permitting the carrying of sufficient fuel for a useful operating range.
A flexible wing construction is one wherein the wing has a central longitudinal keel with leading edge members extending outward and rearward from a forward end of the keel, with lifting surfaces taking the form of flexible panels, usually fabric, of generally a triangular shape secured along the keel and leading edge members and extending therebetween. One disadvantage of the construction is very low wing loading capability, and lack of safe operation under turbulent conditions. Indeed, pilot discomfort is often so pronounced, that comfortable or even safe operation of ultralights must be limited to early morning and late afternoon periods, to avoid thermals, or relatively calm days when wind is low. Otherwise, subjecting these aircraft to gust loads may structurally damage the wing, often with fatal results.
One method of avoiding excessive wing loading is by attaching the wing to the hang cage or fuselage so that the wing is free to pivot about a spanwise axis forward of the aerodynamic center of the wing. Thus, so that the wing is subject to only aerodynamic pitching movements imposed by wing lift and drag. Among the various advantages realized when employing a free wing are alleviation of gust loads, simplicity in fuselage or hang cage design, extension of travel limits for center of gravity, improved fuselage or hang cage attitude trim, and avoidance of hang cage rotation at lift off and landing. Excessive wing loading is also avoided since the aerodynhmic center of pressure of the wing automatically shifts as the wing interacts with changing relative wind (e.g., wind gusts); in other words, the wing pitches up or down to maintain a constant angle of attack to avoid excessive wing loading and enable stall free flight.
For rotatability, free wings must be hinged to the air frame or hang cage and supported thereon in a spanwise or longitudinal plane. However, known hinge construction techniques of which I am aware generally utilize rather complex and expensive hinging mechanisms, such as is disclosed in U.S. Pat. No. 3,361,388 to Girard et al. Furthermore, since the free wing is still subject to bending stress transmitted by the flexible panels to the keel, prior art aircraft of which I am aware generally utilize structurally complex or heavy keels capable of absorbing bending moments, such as a rigid box spar keel member disclosed in U.S. Pat. No. 3,140,842 to Craigo et al or the rigid, solid keel disclosed in the above patent to Girard et al. These prior art keels and the complex mechanisms used for hinging same to the air frame are expensive and unnecessarily increase aircraft weight.
It is accordingly an object of the instant invention to provide an improved free wing especially for ultralight aircraft as well as for conventional aircraft.
It is another object to provide a free wing for ultralight aircraft having improved gust alleviation characteristics.
A further object of the invention is to provide an effective means for hinging a wing having a light weight tubular keel of hollow construction to a hang cage so that the wing is free to rotate in pitch.
Another object of the invention is to provide a free wing wherein bending loads acting upon the keel of an ultralight aircraft are converted into tension and compression loads distributed to other structural parts of the wing using an inexpensive and light weight cable and spreader tube arrangement.
Another object of the invention is to provide a free wing in an ultralight aircraft utilizing a simple, inexpensive yet reliable hinging mechanism for securing the wing to the aircraft.
Yet another object is to provide a free wing in an ultralight aircraft wherein the sole connecting structure between the wing and hang cage is the hinge mechanism permitting rapid detachment of the wing from the hang cage.